Many mobile terminals control output power by controlling a bias voltage applied to an output stage of a radio frequency (RF) power amplifier in the transmit chain. However, as described in commonly owned and assigned U.S. Pat. No. 6,701,138, POWER AMPLIFIER CONTROL, issued Mar. 2, 2004, which is assigned to RF Micro Devices, Inc. of 7628 Thorndike Road, Greensboro, N.C. 27409 and hereby incorporated herein by reference in its entirety, the output power may also be controlled by power control circuitry that controls the supply voltage provided to the power amplifier.
There are two alternatives for the design of the power control circuitry. First, the power control circuitry may be a Low Dropout (LDO) voltage regulator. The LDO voltage regulator includes a series pass element, such as a field effect transistor (FET) coupled in series between a source voltage, such as a battery, and an input terminal of the power amplifier. The supply voltage provided to the power amplifier is controlled by controlling the LDO voltage regulator. However, in order to reduce the output power of the power amplifier, the supply voltage is also reduced. In doing so, the voltage drop across the series pass element is increased, and a larger portion of the power supplied from the battery is dissipated as heat.
The second alternative is for the power control circuitry to be a switching power supply, also known as a switching DC—DC converter. Unlike the LDO voltage regulator, the switching power supply does not include a series pass element designed to drop the output voltage, and therefore avoids the problem of dissipating larger portions of the power supplied from the battery as heat as the supply voltage is decreased. However, the switching power supply requires one or more inductors, which increase the cost of the circuitry.
Due to cost and performance trade-offs, both the LDO voltage regulator and the switching power supply have their place in the market. Thus, there is a need for a single power control circuit that is easily configured as either an LDO voltage regulator or a switching power supply while minimizing the cost of manufacture.